Structural adhesives, in particular structural epoxy adhesives, are useful for generating bonds having a mechanical strength comparable to that achieved by mechanical fasteners. Therefore, structural adhesives can be used to augment or even replace conventional joining techniques such as welding or the use of mechanical fasteners.
Structural epoxy adhesives can be used for bonding a variety of substrates, for example, metal substrates, such as steel and aluminium, or synthetic substrates, such as fiber-reinforced composites. However, to achieve strong bonds, these substrates need to be pre-treated, for example by applying layers of primer compositions to create a good bond between epoxy adhesive and substrate.
The use of primers is economically disadvantageous. Therefore, instead of applying primer layers, adhesion promoters have been added to epoxy compositions. Adhesion promoters for epoxy-based adhesives as known in the art are based on organic silanes, such as described, for example, in Patent Application No. US2009/0297856 to Dohner et al. However, it has been found that the use of silanes as bonding promoters may not give satisfying results on certain substrates, in particular metal or plastic substrates.
Accordingly, there is a need for structural epoxy adhesive compositions that provide good adhesion to substrates, in particular substrates selected from metals such as, for example, steel or aluminium, or plastics. Desirably, such adhesive compositions achieve good bonds of high mechanical strength without requiring primers.